In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Today, there is a progressively increasing trend in dentistry toward the use of automated technologies for treatment planning, virtual procedures, orthodontics, design and manufacturing of dental restorations both in dental offices (chair side) and dental laboratories (lab side). This trend, sometimes called “digital revolution,” is most evident in lab side explosion of CAD/CAM technologies. A number of CAD/CAM systems available to dental laboratories has increased nearly ten-fold in the last decade. Currently, there are over 25 dental CAD/CAM systems and quite a few copy-milling systems using mill blanks in a variety of shapes and sizes. Blank shapes vary from simple geometries such as rectangular, cylindrical or hexagonal to more complex such as smart blanks described in U.S. Pat. No. 6,979,496 which is incorporated by reference herein in its entirety. Their sizes range from about 0.5″ to about 4″ in length or diameter. Mill blanks are available in all 4 types of materials—metals, polymers (resins, plastics), ceramics and composites. Ceramic mill blanks can be divided into three major categories: feldspathic (leucite-based and sanidine or feldspar-based), glass-ceramic (lithium silicate, micaceous, etc.), and crystalline ceramic based such as alumina and/or zirconia (soft-sintered or fully dense). All three ceramic categories as well as composite blanks are already available or soon will be available in a variety of shades. Stocking the necessary inventory of shades for each given type of blank adds to economic pressures on the facility operating a CAD/CAM system.
A conventional 4 inch diameter disk-shaped zirconia blank 100 is illustrated in FIGS. 1-2. As illustrated therein a plurality of milled shapes 110 are formed in the zirconia blank 100. The blank 100 is formed entirely from zirconia, and therefore is quite costly. As illustrated in FIG. 2, use of such blanks 100 to form a plurality of milled shapes 110 results in a significant amount of wasted intervening block area 120 defined between the milling envelopes 112.
While CAD/CAM technology provides dental laboratories with opportunities for improved quality, reproducibility and elimination of human error, most CAD/CAM systems are geared to milling soft-sintered zirconia and thus lacking material selection to be competitive in a supersaturated and fast-paced market. Since the price for a CAD/CAM system, depending on manufacturer and configuration, runs from $50,000 to $500,000 only the largest labs and outsource centers can afford to operate multiple systems to expand their material selection. Most CAD/CAM systems manufacturers do not make their own blocks, rather they purchase them from suppliers such as Ivoclar, Vita or Metoxit, with an established core competency in dental or advanced materials development and manufacturing. Understandably, CAD/CAM materials are fairly expensive adding substantially to CAD/CAM system operating costs. For example, the price of ceramic milling blanks range from about $0.60 to $4.50 per gram of material. Yield per blank as defined in U.S. Pat. No. 6,979,496 is fairly low and most of it goes to waste.
The first CAD/CAM systems comprising milling units for chair side or lab side use such as Cerec (Sirona) and Lava (3M/ESPE) were closed systems wherein mill blanks are attached to a stub retainer, projection, mandrel, holder or carrier body, which have a unique patented geometry as described in U.S. Pat. Nos. 6,485,305 and 6,769,912 and can be also protected by a bar-code, thereby preventing interchangeability with other (CAD/CAM) systems. Variations of a work piece (millable part) on a stub assembly are also described in U.S. Pat. Nos. 7,214,435, 6,669,875, 6,627,327, 6,482,284, 6,224,371, 6,991,853 and 6,660,400. With advent of the open architecture systems, blank interchangeability between systems has become not only possible but extremely desirable. While the market is currently dominated by closed systems, the market penetration of open systems is steadily increasing. From 25 commercial CAD/CAM systems at least 5 or 6 are utilizing the same D-250 dental 3D scanner and DENTALDESIGNER™ dental CAD software (3Shape A/S, Copenhagen, Denmark). In an open architecture system, the blanks are not bar-code protected and any blank can be used as long as it fits the existing housing (blank holder, chuck, collect, support) of the milling unit.
Not all types of blanks can be economically produced in any shape and size. For example, zirconia and alumina blocks can be formed in any given shape and size to meet the demand for larger cases that can be milled from larger blanks. On the other hand, large feldspathic and glass-ceramic blanks are not so desirable due to a number of mechanical and economic constraints.
U.S. Patent Application 2006/0115794 appears to teach a system for continuous production of prosthodontic pieces such as crown cores, crowns or the like. The system utilizes turning and milling on a live center computer numerical control CNC machine of a zirconia rod stock that is automatically fed into the machine. Multiple pieces are cut one after another from the continuous rod stock. This patent application further appears to teach utilization of multiple machines wherein each machine is fed a rod stock of a different shape and/or size. A central control unit obtains specifications for a piece that is to be cut and selects the machine on which the piece is to be made by determining the rod stock that will require the least amount of cutting. In addition to the above mentioned economical and processing difficulties of fabricating and milling long rod stock from materials other than fully dense zirconia, considering the cost of the CNC machine, it is far more advantageous to enable one machine to mill all cases than to have many machines, each dedicated to a certain type of case.
U.S. Pat. No. 7,234,938 appears to disclose the multi-blank holder or workpiece receiver constructed as an elongated strip with multiples bores in it for embedding a plurality of identical blanks or workpieces. The invention relates to a milling/grinding machine, wherein, the workpiece receiver or mill blank holder has a plurality of bores arranged along its longitudinal axis, for receiving the workpieces or blanks. This invention also comprises a moldable embedding material disposed within the through-bore for retaining the workpiece within the through-bore. It further teaches a milling/grinding machine, comprising an embedding device for the automatic embedding of the workpiece in the workpiece receiver.
U.S. Patent Application 2006/0106485 describes the use of a virtual blank corresponding to a physical blank being processed to form a plurality of manufacturing features. This application further teaches virtual machining of each manufacturing feature of the plurality of manufacturing features into the virtual blank wherein each manufacturing feature exhibits an associative relationship with the coordinate system. Manufacturing instructions are generated to create the actual part by machining the plurality of manufacturing features into the blank. Such methods were pioneered in the automotive industry and described in U.S. Pat. Nos. 6,775,581; 7,024,272; 7,110,849 and U.S. Patent Application 2006/0106485, incorporated by reference herein in their entirety. It is also described in the white paper: Horizontal Modeling & Digital Process Design. The approach of electronically designing an article comprising an assembly of components is described in US Application 2007/0136031 incorporated by reference herein in its entirety. Again, this disclosure is not related to dentistry.
Thus, a need exists in the art for enabling blank interchangeability, maximizing yield per blank, and reducing material waste, to maximize the system's versatility, selection of materials and efficiency of operation. There is also a desire to reduce inventory of blanks thus reducing operating costs associated with commercial CAD/CAM systems.